Optical and radio variability of the BL Lacertae object AO 0235+16: A possible 5-6 year periodicity

The BL Lacertae object AO 0235+16 is well known for its extreme optical and radio variability. New optical and radio data have been collected in the l ast four years by a wide international collaboration, which confirm the int ense activity of this source: on the long term, overall variations of 5 mag in the R band and up to a factor 18 in the radio fluxes were detected, whi le short-term variability up to 0.5 mag in a few hours and 1.3 mag in one d ay was observed in the optical band. The optical data also include the resu lts of the Whole Earth Blazar Telescope (WEBT) first-light campaign organiz ed in November 1997, involving a dozen optical observatories. The optical s pectrum is observed to basically steepen when the source gets fainter. We h ave investigated the existence of typical variability time scales and of po ssible correlations between the optical and radio emissions by means of vis ual inspection and Discrete Correlation Function (DCF) analysis. On the lon g term, the autocorrelation function of the optical data shows a double-pea ked maximum at 4100-4200 days (11.2-11.5 years), while a double-peaked maxi mum at 3900-4200 days (10.7-11.5 years) is visible in the radio autocorrela tion functions. The existence of this similar characteristic time scale of variability in the two bands is by itself an indication of optical-radio co rrelation. A further analysis by means of Discrete Fourier Transform (DFT) technique and folded light curves reveals that the major radio outbursts re peat quasi-regularly with a periodicity of similar to 5.7 years, i.e. half the above time scale. This period is also in agreement with the occurrence of some of the major optical outbursts, but not all of them. Visual inspect ion and DCF analysis of the optical and radio light curves then reveal that in some cases optical outbursts seem to be simultaneous with radio ones, b ut in other cases they lead the radio events. Moreover, a deep inspection o f the radio light curves suggests that in at least two occasions (the 1992- 1993 and 1998 outbursts) flux variations at the higher frequencies may have led those at the lower ones.